Torsion Pendulum

The torsion pendulum was used by Henry Cavendish to measure the gravitational force between two masses. A rotatable rod with two small balls at its ends hangs from a wire; it has a mirror in the middle that deflects a beam of light. There are two larger fixed balls that make an angle with the rod. The two pairs of balls attract each other, causing the wire to turn; the angle measures the force. A similar device was used later by Charles Augustin de Coulomb to discover the inverse square law for the force between two electric charges.